EP3482174B1 - Electromagnetic drive and receive unit for a field device in the automation technology - Google Patents

Description

The invention relates to an electromechanical converter unit for a field device in automation technology and a device for determining and / or monitoring at least one process variable of a medium in a container with an electromechanical converter unit according to the invention. The process variable is given, for example, by the fill level or the flow rate of the medium or also by its density or viscosity. The medium is located, for example, in a container, a tank, or in a pipeline.

A wide variety of field devices are used in automation technology to determine and / or monitor at least one process variable, in particular a physical or chemical process variable. These are, for example, level measuring devices, flow measuring devices, pressure and temperature measuring devices, pH redox potential measuring devices, conductivity measuring devices, etc., which record the corresponding process variables level, flow, pressure, temperature, pH value or conductivity, etc. The respective measuring principles are known from a large number of publications.

A field device typically includes at least one sensor unit that comes into contact with the process at least partially and at least temporarily, and an electronics unit which is used, for example, to acquire, evaluate and / or feed signals. In the context of the present application, field devices are in principle all measuring devices that are used close to the process and that deliver or process process-relevant information, including remote I / Os, radio adapters or generally electronic components that are arranged on the field level. A large number of such field devices are manufactured and sold by the applicant.

Electromechanical converter units are used in a number of corresponding field devices. For example, vibronic sensors such as vibronic fill level or flow measuring devices may be mentioned here, but they are also used in ultrasonic fill level measuring devices or flow measuring devices. To deal separately and in detail with each type of field device and its underlying measuring principle, for which an electromechanical converter unit according to the invention can be used, would go beyond the scope of the present application. Therefore, for the sake of simplicity, the following description is restricted where reference is made to specific field devices, for example to fill level measuring devices with an oscillatable unit. Corresponding field devices are from sold by the applicant under the name LIQUIPHANT or SOLIPHANT, for example. The underlying measurement principles are basically known.

The oscillatable unit of such a level measuring device, also referred to as a vibronic sensor, is, for example, an oscillating fork, a single rod or a membrane. This oscillatable unit is excited during operation by a drive / receiver unit, usually in the form of an electromechanical converter unit, for example in the form of a piezoelectric, electromagnetic or magnetostrictive drive / receiver unit, by means of an electrical excitation signal to produce mechanical vibrations. Conversely, the drive / receiver unit can receive the mechanical vibrations of the mechanically vibratable unit and convert them into an electrical received signal. The drive / receiver unit can either be a separate drive unit and a separate receiver unit, or it can be a combined drive / receiver unit.

A wide variety of both analog and digital processes have been developed to excite the mechanically oscillatable unit. In many cases, the drive / receiver unit is part of a feedback electrical oscillating circuit, by means of which the mechanically oscillatable unit is excited into mechanical oscillations. For example, the resonant circuit condition, according to which the gain factor gemäß 1 and all phases occurring in the resonant circuit result in a multiple of 360 °, must be fulfilled for a resonant oscillation. As a result, a certain phase shift between the excitation signal and the received signal must be guaranteed. A wide variety of solutions have become known for this. In principle, the phase shift can be set, for example, by using a suitable filter, or it can be regulated to a predefinable phase shift, the setpoint value, by means of a control loop. From the DE102006034105A1 For example, it has become known to use an adjustable phase shifter. The additional integration of an amplifier with an adjustable gain factor for additional regulation of the oscillation amplitude was, however, in the DE102007013557A1 described. the DE102005015547A1 suggests the use of an Allpass. The phase shift can also be set by means of a so-called frequency search, for example in the DE102009026685A1 , DE102009028022A1 , and DE102010030982A1 disclosed. The phase shift can, however, also be regulated to a specifiable value by means of a phase-locked loop (PLL). An excitation method based on this is the subject of DE00102010030982A1 .

Both the excitation signal and the received signal are characterized by their frequency, amplitude and / or phase. Changes in these sizes are then usually used Determination of the respective process variable is used, such as a specified fill level of a medium in a container, or the density and / or viscosity of a medium. In the case of a vibronic point level switch for liquids, a distinction is made, for example, between whether the oscillatable unit is covered by the liquid or whether it oscillates freely. These two states, the free state and the covered state, are recognized, for example, on the basis of different resonance frequencies, that is to say a frequency shift, or on the basis of a damping of the oscillation amplitude.

The density and / or viscosity, in turn, can only be determined with such a measuring device if the oscillatable unit is covered by the medium. From the DE10050299A1 , the DE102006033819A1 and the the DE102007043811A1 It has become known to determine the viscosity of a medium using the frequency-phase curve (φ = g (f)). This procedure is based on the dependency of the damping of the oscillatable unit on the viscosity of the respective medium. In order to eliminate the influence of the density on the measurement, the viscosity is determined using a frequency change caused by two different values for the phase, i.e. using a relative measurement. To determine and / or monitor the density of a medium, however, according to the DE10057974A1 the influence of at least one disturbance variable, for example the viscosity, on the oscillation frequency of the mechanically oscillatable unit is determined and compensated for. In the DE102006033819A1 is also described to set a predeterminable phase shift between the excitation signal and the received signal, at which effects of changes in the viscosity of the medium on the mechanical vibrations of the mechanically vibratable unit are negligible. With this phase shift, an empirical formula can be set up to determine the density.

As already mentioned, the drive / receiver unit is usually designed as an electromechanical converter unit. It often includes at least one piezoelectric element in a wide variety of configurations. Using the piezoelectric effect, a comparatively high degree of efficiency, i.e. the efficiency of converting electrical into mechanical energy, can be achieved. Corresponding piezoceramic materials based on PZT (lead zirconate titanate) are normally suitable for use at temperatures of up to 300 ° C. There are piezoceramic materials that retain their piezoelectric properties even at temperatures above 300 ° C; However, these have the disadvantage that they are significantly more ineffective than the PZT-based materials. In addition, there are significant differences between the thermal expansion coefficients of metals and ceramic materials, which can be disadvantageous, especially at high temperatures.

Due to its function as a force transmitter, a non-positive connection of the respective piezoelectric element with a membrane of the sensor, which is at least part of the oscillatable unit, must always be guaranteed. In particular, at high temperatures, however, there is an increased increase in mechanical stresses which can break the piezoelectric element and consequently lead to total failure of the sensor.

An alternative that can be more suitable for use at high temperatures is represented by so-called electromagnetic drive / receiving units, as for example in the publications WO 2007/113011 and WO 2007/114950 A1 described. The conversion of electrical energy into mechanical energy takes place via a magnetic field. A corresponding electromechanical converter unit comprises at least one coil and one permanent magnet. An alternating magnetic field penetrating the magnet is generated by means of the coil, and a periodic force is transmitted to the oscillatable unit via the magnet. This periodic force is usually transmitted in a manner similar to the principle of a plunger, which is placed in the middle of the membrane.

Since an electromagnetic drive / receiver unit does not require a non-positive connection with the membrane of the oscillatable unit, these can be used in an extended temperature range, in particular between -200 ° C and 500 ° C, compared to piezoelectric transducer units. However, due to the lack of a non-positive connection, the efficiency is usually significantly lower than with piezoelectric drive / receiver units. Although an electromagnetic drive / receiver unit can develop relatively high forces in the area of the diaphragm, the deflection of the tuning fork is comparatively small due to the non-positive connection between the diaphragm and the drive. Consequently, more energy is required for an electromagnetic drive / receiver unit in comparison to a piezoelectric drive / receiver unit, which makes the use of a corresponding sensor in areas at risk of explosion problematic.

An electromechanical converter unit with increased efficiency is in the DE102015104533A1 described, to which reference is made in full in the following.

In the DE1773815 and GB2185575A vibronic sensors are disclosed in each case, in which rods are used to generate and detect vibrations. the EP0949489A1 describes a vibronic sensor in the form of a single rod, in which a rod-shaped vibration device is attached to the membrane in the center of the rod.

Based on the prior art, the object of the present invention is to propose an electromagnetic drive / receiver unit or an electromechanical converter unit with at least one coil and a magnet, which is characterized by an increased efficiency compared to the prior art.

• eine in mechanische Schwingungen versetzbare Membran,
• zumindest drei senkrecht zu einer Grundfläche der Membran an der Membran kraftschlüssig befestigte Stangen,
• ein Gehäuse, wobei die Membran zumindest einen Teilbereich einer Wandung des Gehäuses bildet, und wobei die Stangen ins Gehäuseinnere hineinreichen,
• zumindest drei Magnete, wobei an jeder der zumindest drei Stangen in dem der Membran abgewandten Endbereich jeweils einer der Magnete befestigt ist, und
• eine Spule mit Kern, welche innerhalb des Gehäuses oberhalb der Magnete befestigt ist, und welche Spule mit einem elektrischen Wechselstromsignal beaufschlagbar ist,

wobei die Spule dazu ausgestaltet ist, ein Magnetfeld zu erzeugen, welches Magnetfeld die Stangen mittels der Magnete in mechanische Schwingungen senkrecht zur Längsachse der beiden Stangen versetzt,
wobei die Stangen derart an der Membran befestigt sind, dass aus den Schwingungen der Stangen Schwingungen der Membran resultieren, und wobei zumindest eine der Stangen im Wesentlichen an einem Ort entlang der Grundfläche der Membran an der Membran befestigt ist, an welchem Ort die zweite Ableitung der Auslenkung der Membran aus einer Ruheposition als Funktion des Orts auf der Grundfläche der Membran im Wesentlichen null ist.This object is achieved according to the invention by an electromechanical converter unit for a field device of automation technology according to claim 1, comprising at least

• a membrane that can be set into mechanical vibrations,
• at least three rods non-positively attached to the membrane perpendicular to a base surface of the membrane,
• a housing, wherein the membrane forms at least a portion of a wall of the housing, and wherein the rods extend into the interior of the housing,
• at least three magnets, one of the magnets being fastened to each of the at least three rods in the end region facing away from the membrane, and
• a coil with a core, which is fastened inside the housing above the magnets, and which coil can be supplied with an electrical alternating current signal,

wherein the coil is designed to generate a magnetic field, which magnetic field sets the rods in mechanical vibrations perpendicular to the longitudinal axis of the two rods by means of the magnets,
wherein the rods are fastened to the membrane in such a way that vibrations of the membrane result from the vibrations of the rods, and wherein at least one of the rods is fastened to the membrane substantially at a location along the base of the membrane, at which location the second derivative of the Deflection of the membrane from a rest position as a function of the location on the base of the membrane is essentially zero.

The conversion of electrical into mechanical energy takes place via an alternating magnetic field, which is generated by means of the coil with core. A magnetic field prevailing at a fixed, selectable point in time in the area of the rods causes the rods attached to one side to the membrane to be deflected in the area facing away from the membrane, in which the magnets are attached to the rods. Rods are thus caused to vibrate by the alternating magnetic field, the vibratory movement of the rods taking place transversely or perpendicularly to their longitudinal axis. The rods behave like their own mechanical resonator. Due to the connection, in particular a non-positive connection, of the rods with the membrane, the oscillatory movement of the rods is also transmitted to the membrane, which thus also executes an oscillatory movement.

According to the invention, at least one of the rods is attached to the membrane essentially at a location along the base of the membrane, at which location the second derivative of the deflection of the membrane from a rest position as a function of the location on the base is essentially zero. The locations along the base area of the membrane at which the second derivative of the deflection of the membrane from a rest position is essentially zero depends, among other things, on the respective vibration mode of the membrane.

At least one of the rods is therefore attached to the membrane essentially in the region of a turning point of the deflection from the rest position as a function of the location on the membrane. The rest position of the membrane is that position in which no resultant force acts on the membrane, in which the membrane is not bent, but is essentially planar.

By positioning the rods according to the invention, a particularly efficient transmission of the vibratory movements of the rods to the membrane is achieved. The energy required to operate the converter unit according to the invention is correspondingly advantageously minimized compared to equivalent electromechanical converter units in which, however, the rods are attached in other areas along the base of the membrane.

The electromechanical converter unit according to the invention is also ideally suited for use in an extended temperature range, in particular for use at high temperatures. The maximum permissible temperature depends, for example, on the material chosen for the magnets. Since the rods are directly connected to the membrane in one end area and form their own mechanical resonator, the efficiency of an electromechanical transducer unit according to the invention is higher than the variants mentioned in the introduction for electromagnetic drive / receiver units based on the principle of a plunger from the prior art clearly increased. Nevertheless, the structural design of an electromechanical converter unit according to the invention is comparatively simple. The magnets are, for example, so-called Alnico magnets. Alnico magnets are sometimes also referred to as steel magnets. These are alloys made of iron, aluminum, nickel, copper and cobalt, from which permanent magnets are made using casting techniques or sintering processes. Among other things, such magnets are characterized by a high remanence flux density (approx. 0.6-1.3T) and a high Curie temperature of 700-850 ° C, which allows applications in a temperature range of at least up to 500 ° C. So-called rare earth magnets, which essentially consist of ferrous metals and rare earth metals, represent an interesting alternative. For example, samarium cobalt, hereinafter referred to as SmCo magnet, can currently be used at temperatures of up to 350 °; In research, however, efforts are being made to achieve operating temperatures of more than 500 ° C. It goes without saying that, however, other magnets can also be used for the present invention, which likewise fall under the present invention. The magnets are designed, for example, rod-shaped. The shape of the magnets is preferred In one embodiment of the invention, however, it is also adapted to the shape of the coil with core, in particular such that a surface of the magnets facing the coil with core is maximized as possible and the distance between the respective magnet and the coil with core is selected to be as small as possible.

According to a preferred embodiment of the converter unit according to the invention, at least one of the rods is attached to the membrane essentially along a circular line running around the center point of the base area of the membrane. This choice is particularly advantageous for vibrations of the membrane in the fundamental oscillation mode, in which the center point of the membrane experiences the greatest deflection. However, this configuration is also suitable for higher oscillation modes in which the center point of the membrane experiences the greatest deflection. For higher vibration modes, there are increasingly nodal lines along the base of the membrane.

One embodiment of the invention includes that the number of rods is an even number, the rods being arranged symmetrically along the circular line around the center point of the membrane.

Alternatively, the number of rods is an odd number, the rods being arranged equiangularly along the circular line around the center of the membrane.

It is advantageous if the coil with the core is arranged essentially above the center point of the base area of the membrane.

For example, the core of the coil can be part of a pot-shaped armature unit, which armature unit has a base and a peripheral wall, a connecting piece being attached starting from the base and pointing centrally into the interior of the armature unit, the connecting piece forming the core of the coil, and the circumferential wall serves as a magnetic field feedback. The circumferential wall then extends, for example, to the rods which should not touch the anchor unit. This refinement offers structural advantages, on the one hand, since both the coil core and a field return can be provided in one piece in the form of the armature unit. The field feedback also ensures magnetic shielding, which results in increased immunity to interference. The anchor unit is preferably made of a material with high magnetic permeability, in particular iron, cobalt, or cobalt iron, or consists of a metallic glass. With regard to a high magnetic permeability, ferromagnetic materials with at least μ> 100 are particularly suitable. For example, the permeability µ of cobalt iron is in the range µ _cobalt iron ≈10000-150000, that of cobalt in the range µ _cobalt ≈100-200 and for iron µ _iron ≈300-10000 applies. In particular, it is advantageous if the material for the anchor unit has the lowest possible hysteresis. The hysteresis should be at least so small that the material can follow the constant magnetization reversal according to the frequency of the excitation signal.

Ferromagnetic materials are particularly suitable for use at high temperatures. If, on the other hand, the requirement for usability for particularly high temperatures is not the main focus, metallic glasses, whose magnetic permeability is typically in the range µ _Metglas ≈1500-4000, are of interest because they have a particularly low hysteresis and the associated low losses during magnetization reversal , exhibit.

A preferred embodiment provides that each of the magnets has essentially the same distance from the coil with the core. It is advantageous if the distance between each of the magnets and the coil with core is less than 2mm. This information relates to a point in time at which there is currently no magnetic field. So that all rods are set into mechanical vibrations in the same way, the distance between each of the rods, or the magnets attached to the ends of the rods, and the coil with core should be essentially the same.

For the highest possible efficiency of the energy transfer from the coil with core to the rods, it is still important to choose this distance as small as possible. However, it is always important to ensure that the magnets do not touch the coil with core.

If the core of the coil is designed as part of a cup-shaped armature unit, the magnets reach into the cup-shaped armature unit without contact, so that if there is no magnetic field, they are at the same distance from the coil. In this way, the two magnets are completely enveloped by the magnetic field feedback.

• eine Sensoreinheit mit zumindest einer erfindungsgemäßen elektromechanischen Wandlereinheit, und
• eine Elektronikeinheit,

wobei die elektromechanische Wandlereinheit dazu ausgestaltet ist, die Sensoreinheit mittels eines elektrischen Anregesignals in Form eines elektrischen Wechselstromsignals, mit welchem die Spule beaufschlagt ist, zu mechanischen Schwingungen anzuregen, und die mechanischen Schwingungen der Sensoreinheit zu empfangen und in ein elektrisches Empfangssignal in Form eines elektrischen Wechselstromsignals umzuwandeln, und wobei die Elektronikeinheit dazu ausgestaltet ist, das Anregesignal ausgehend vom Empfangssignal zu erzeugen, und die zumindest eine Prozessgröße zumindest anhand des Empfangssignals zu bestimmen. Die erfindungsgemäße elektromechanische Wandlereinheit ist also im Prinzip eine Antriebs-/Empfangseinheit der erfindungsgemäßen Vorrichtung, oder zumindest ein Teil davon. Die elektromechanische Wandlereinheit kann auch entweder in Form einer quasi separaten Antriebseinheit oder Empfangseinheit eingesetzt werden.The object according to the invention is further achieved by a device for determining and / or monitoring at least one process variable of a medium in a container comprising at least one

• a sensor unit with at least one electromechanical converter unit according to the invention, and
• an electronics unit,

wherein the electromechanical converter unit is configured to excite the sensor unit to mechanical vibrations by means of an electrical excitation signal in the form of an electrical alternating current signal, with which the coil is applied, and to receive the mechanical vibrations of the sensor unit and convert it into an electrical received signal in the form of an electrical one To convert alternating current signal, and wherein the electronics unit is configured to generate the excitation signal based on the received signal, and to determine the at least one process variable at least on the basis of the received signal. The electromechanical converter unit according to the invention is therefore in principle a drive / receiving unit of the device according to the invention, or at least a part thereof. The electromechanical converter unit can also be used either in the form of a quasi-separate drive unit or receiving unit.

The sensor unit furthermore preferably comprises a unit that can vibrate, in particular mechanically. In this case, the device according to the invention is a vibronic sensor.

The length L of the two rods is advantageously chosen such that L = nλ / 2 + λ / 4, where λ is the wavelength of the waves propagating along the propagating rods and n is a natural number. The length of the rods is therefore selected in accordance with a desired excitation frequency of the excitation signal and in accordance with their vibration properties. Furthermore, by adjusting the length of the rods and also by designing a housing of the device, a temperature decoupling can be achieved, which results from the spatial separation, in particular the magnets and coil with core, from the process. The device according to the invention can therefore be specifically adapted to the respective process requirements. This is done on the one hand by choosing the length of the rods. In addition, the housing can also be made of a material which is characterized by good thermal insulation, so that it also has the function of a temperature spacer tube. A greater distance from the membrane, i.e. a spatial separation of the magnets and coil from the membrane, causes temperature decoupling. In this way, the permissible temperature range can be further expanded beyond that which is defined by the magnets. It should be noted here, however, that the longer the rods are, the lower the efficiency of the power transmission. It is therefore always necessary to weigh up the desired permissible temperature range and the desired efficiency.

One embodiment provides that the oscillatable unit comprises at least a partial area of the membrane, or at least a partial area of the membrane and at least one oscillating rod attached to it. In this case, the oscillatable unit is a membrane, a single rod or an oscillating fork.

On the one hand, the membrane can be made in one piece. In the event that the correspondingly configured device comprises an oscillatable unit, the membrane is then on the one hand assigned to the electromagnetic transducer unit, but at the same time forms at least part of the vibratable unit. On the other hand, an embodiment includes that the membrane has two frictionally interconnected partial areas, a first partial area being assigned to the electromagnetic transducer unit and a second partial area being assigned to the oscillatable unit. The connection between the two partial areas can then be established, for example, by means of a soldered, welded or glued connection.

The oscillatable unit is arranged, for example, in a defined position within the container in such a way that it dips into the medium up to a determinable immersion depth. In particular, the process variables viscosity and / or density can be determined in this way.

It is advantageous if the process variable is given by a fill level or the flow rate of the medium in the container, or by the density or the viscosity of the medium.

According to a preferred embodiment of the present invention, the vibrating unit is a vibrating fork with two vibrating rods, the electromechanical transducer unit comprising four rods, and wherein two of the four rods of the electromechanical converter unit fastened to the membrane and the two vibrating rods fastened to the membrane are mirror-symmetrical with respect to each other the plane perpendicular to the longitudinal axis through the rods and / or vibrating rods are arranged opposite one another. In each case a vibrating rod of the vibratable unit and a rod of the electromechanical converter unit thus run essentially along the same imaginary line parallel to their two longitudinal axes. In particular, these two rods and vibrating rods are arranged in such a way that they are at the same distance from the center point of the base surface of the membrane perpendicular to the longitudinal axis of the rods and vibrating rods. This symmetrical arrangement in the case of a vibronic sensor with a vibrating fork as a vibratable unit achieves a particularly high level of efficiency.

The oscillating rods, rods and the membrane form a coupled oscillating system, the coupling being determined by the membrane. For the example of a vibratory unit in the form of a vibrating fork, for example, the two vibrating rods and the membrane form a first mechanical resonator, the two rods of the electromechanical transducer unit with the membrane opposite the vibrating rods form a second resonator, and the other two rods and the membrane form a third resonator . The frequency of the excitation signal is preferably chosen so that the first and second resonators vibrate in an antisymmetrical vibration mode based on the plane through the membrane perpendicular to the longitudinal axis of the rods and / or vibrating rods. In the, in principle formed from three resonators, There are basically three resonance frequencies in the oscillating system. This is related to Fig. 5 described in more detail.

An alternative preferred embodiment of the present invention also provides that the oscillatable unit is an oscillating fork with two oscillating rods. However, the electromechanical transducer unit comprises three rods, the three rods being arranged in the corner points of an equiangular triangle running around the center point M of the membrane, in such a way that the connecting line between two of the three rods runs parallel to a connecting line between the two oscillating rods.

An arrangement of at least one of the rods in a region of the membrane, at a location at which the second derivative of the deflection of the membrane from a rest position as a function of the location along the base area is essentially zero, ensures a particularly high efficiency for the transmission of the Vibrations from the rods to the membrane and possibly to an oscillating unit. The efficiency increases with the number of rods used. However, there are limits to maximizing the number of rods, inter alia, due to the space available within the housing of the device. It should be pointed out that the frequencies of the oscillating movements can be influenced in particular by adjusting the length and / or rigidity of the rods.

• Fig. 1: (a) eine schematische Skizze eines vibronischen Sensors und (b) eine perspektivische Ansicht einer Schwinggabel gemäß Stand der Technik,
• Fig. 2: (a) eine Seitenansicht einer erfindungsgemäßen elektromechanische Wandlereinheit, sowie eine Ansicht für eine bevorzugte Anordnung der Stangen entlang der Membran im Falle von (b) drei und (c) vier Stangen,
• Fig. 3 ein vibronisches Füllstandsmessgerät mit einer Schwinggabel als schwingfähiger Einheit und einer erfindungsgemäßen elektromechanischen Wandlereinheit mit vier Stangen,
• Fig. 4: (a) Biegelinien der Membran für das vibronisches Füllstandsmessgerät aus Fig. 3, sowie bevorzugte Anordnungen der Stangen entlang der Grundfläche der Membran im Falle von (b) drei und (c) vier Stangen, und
• Fig. 5 ein Frequenzspektrum eines vibronischen Sensors mit einer schwingfähigen Einheit in Form einer Schwinggabel und einer elektromechanischen Wandlereinheit mit vier Stangen.

The invention and its advantageous refinements are illustrated below with reference to the figures Figures 1-5 described in more detail. It shows:

• Fig. 1 : (a) a schematic sketch of a vibronic sensor and (b) a perspective view of a tuning fork according to the prior art,
• Fig. 2 : (a) a side view of an electromechanical transducer unit according to the invention, as well as a view for a preferred arrangement of the rods along the membrane in the case of (b) three and (c) four rods,
• Fig. 3 a vibronic level measuring device with a vibrating fork as a vibratable unit and an electromechanical transducer unit according to the invention with four rods,
• Fig. 4 : (a) Bend lines of the membrane for the vibronic level measuring device Fig. 3 , and preferred arrangements of the rods along the base of the membrane in the case of (b) three and (c) four rods, and
• Fig. 5 a frequency spectrum of a vibronic sensor with a vibratory unit in the form of a vibrating fork and an electromechanical transducer unit with four rods.

In Fig. 1a a vibronic level measuring device 1 is shown. A sensor unit 2 with a mechanically oscillatable unit 3 in the form of an oscillating fork is partially immersed in a medium 4 which is located in a container 5. The oscillatable unit 3 is excited to mechanical oscillations by means of the drive / receiver unit 6, usually an electromechanical converter unit, and can for example be a piezoelectric stack or bimorph drive, but also an electromagnetic or magnetostrictive drive / receiver unit. It goes without saying, however, that other configurations of a vibronic level measuring device are also possible. Furthermore, an electronic unit 7 is shown, by means of which the signal acquisition, evaluation and / or feeding takes place.

Figure 1b shows again a more detailed view of a vibratable unit 3 in the form of a vibrating fork, such as is used for the LIQUIPHANT, for example. A membrane 8 and an oscillating element 9 connected to it can be seen. The oscillating element 9 has two oscillating rods 10a, 10b, on each of which a paddle 11a, 11b is formed at the end. During operation, the tuning fork performs 3 vibratory movements in accordance with the vibration mode with which it is excited. Each of the two oscillating rods 10a, 10b behaves essentially like a so-called flexural oscillator. In the fundamental oscillation mode, the two oscillating rods 10a, 10b oscillate, for example, out of phase with one another.

In Fig. 2a a schematic side view of an electromechanical converter unit 12 according to the invention is shown. In this embodiment, the electromechanical converter unit 12 displays the drive / receiver unit 6 of the measuring device Fig. 1a represent.

A membrane 8, 14 is introduced into the lower wall of a housing 13. The housing 13 therefore ends with the membrane 8, 14 on this side face. In this example, the housing 13 is cylindrical and the membrane 8, 14 is disc-shaped with a circular base area A. It goes without saying, however, that other geometries are also conceivable and fall under the present invention. Three rods 15a, 15b, 15c are attached to the membrane 8, 14 perpendicular to the base area A of the membrane 8, 14 and extending into the interior of the housing 13. In particular, this is a non-positive connection. The base area A of the membrane 8, 14 then lies in a plane perpendicular to the longitudinal direction of the rods 15a, 15b, 15c. For example, the rods 15a, 15b, 15c are arranged equiangularly along an imaginary circular line around the center point M of the base area A of the membrane 8, 14.

In the end region of the rods 15a, 15b, 15c facing away from the membrane 8, 14, a magnet, in particular a SmCo or Alnico magnet, 16a, 16b, 16c, is fastened. The magnets are preferably all aligned or oriented in the same way. In the case of an even number of bars, for example in the case of 4 bars, as in Figure 2c sketched, however, the magnets 16a-16d can also be aligned in the same pairs in the same way.

A coil 17 with a core 18 is arranged above the magnets 16a, 16b, 16c. The rods 15a-15c with the magnets 16a-16c do not touch the coil 17 and the core 18. The coil 17 is subjected to an alternating current signal in continuous operation to generate an alternating magnetic field. Because of this alternating field, the rods 15a-15c are moved horizontally via the magnets 16a-16c, i. H. perpendicular or transverse to their longitudinal axis, deflected in such a way that they are made to vibrate. On the one hand, the rods 15a-15c then have a leverage effect, by means of which the bending of the rods 15a-15c produced by the horizontal deflection is transmitted to the membrane 8, 14 in such a way that the membrane 8, 14 is caused to vibrate. On the other hand, the combination of the two rods 15a-15c and the membrane 8, 14 is a separate resonator. The excitation of the membrane 8, 14 to mechanical vibrations thus takes place by means of an alternating magnetic field.

In this exemplary embodiment, the core 18 of the coil 17 is part of a pot-shaped armature unit 19 with a base 20 and a peripheral wall 21, without loss of generality. For example, the base 20, like the base area A of the membrane 8, 14, can have a circular cross-sectional area. From the bottom 20 of the cup-shaped armature unit 19, the core 18 of the coil 17 extends centrally in the form of a connecting piece into the interior of the armature unit 19. In this case, the peripheral wall 21 then has the function of a magnetic field return. The anchor unit 19 is preferably made of a material with a high magnetic permeability, in particular iron, cobalt, or a metallic glass.

According to the invention, at least one of the rods 15a-15c is essentially attached to a location along the base of the membrane 8.14 on the membrane 8.14, at which location the second derivative of the deflection of the membrane 8.14 from a rest position as a function of the location is essentially zero on the base. In the case of a circular diaphragm 8, 14 with the base area A, which oscillates in the fundamental mode, this area is essentially defined by a circular line 22 running around the center point M of the diaphragm 8, 14, as in FIG Figure 2b shown, given. In the case of higher oscillation modes, nodal lines increasingly form along the membrane 8, 14, so that, depending on the respective oscillation mode, several areas can also exist along the base area A of the membrane 8, 14 for which the second derivative of the deflection is essentially zero.

Preferred positions of the rods 15a-15d along the base area of a membrane 9 with a circular base area A for different numbers of rods 15a-15d are shown in FIG Fig. 2b and Figure 2c shown. According to the exemplary embodiment Figure 2b the electromechanical converter unit 12 comprises three rods 15a, 15b, 15c, each indicated by a circle, which are arranged in the area of the circular line 22 at the same angle with respect to the circumference of the circle. In contrast, an embodiment with four rods 15a-15d arranged along the circular line 22, with two rods 15a and 15b or 15c and 15d lying opposite one another, is shown in FIG Figure 2c shown.

In Fig. 3 Finally, a vibronic fill level measuring device with an oscillatable unit 3 as in FIG Figure 1b and an electromechanical converter unit 12 as in FIG Fig. 2 , but shown with four rods 15a-15d (rod 15d not visible in this illustration). The same reference symbols are therefore not discussed again in the following. In this example, the membrane 8.14 of the electromagnetic converter unit is also the membrane 8 of the vibrating fork 3. It is therefore a one-piece membrane 8.14 which is assigned to both the vibratable unit 3 and the electromechanical converter unit 12. It goes without saying, however, that the membrane 8, 14 can also be made of two frictionally interconnected partial areas 8 and 14 in another embodiment, the first partial area 8 being assigned to the electromechanical transducer unit 12 and the second partial area 14 being assigned to the oscillatable Unit 3.

The two oscillating rods 10a, 10b and two of the four rods 15a, 15b are preferably attached to the membrane 8, 14 in such a way that one rod 15a, 15b and one oscillating rod 10a, 10b each along the same longitudinal axis, that is, the axis perpendicular to the base A through the membrane 8, 14. The two longitudinal axes intersect the plane parallel to the membrane 8, 14 at the same distance from the center M of this area A. This symmetrical arrangement enables increased efficiency to be achieved.

Such an arrangement is a coupled resonator system. For the example with four rods 15a-15d, the two vibrating rods 10a, 10b of the vibratable unit 3 with the membrane 8, 14 form a first mechanical resonator, while the two rods 15a, 15b or 15c, 15d with the membrane 8, 14 each form a second and third mechanical resonator, respectively. All three resonators are mechanically coupled to one another via the membrane 8, 14, the coupling being adjustable via the design of the membrane 8, 14. For example, the coupling can be influenced by the thickness or the material of the membrane 8, 14, but also by the respective connection to the oscillating rods 10a, 10b or rods 15a-15d. In such a resonator system, several oscillation modes with different resonance frequencies occur, which is shown below with reference to FIG Fig. 4 and Fig. 5 is explained. It should be pointed out that, on the other hand, in the event that no oscillatable unit 3 is assigned to the electromechanical converter unit 12, the four rods 15a-15d usually form a single resonator.

The presence of a mechanically oscillatable unit 3 in the form of an oscillating fork on the oscillating membrane 8, 14, as in FIG Fig. 2 shown, thus leads to a changed vibration behavior of the device 1. This is for example based on the in Figure 4a The bending lines shown of the membrane 8, 14 from its center point M to the edge along the two lines m and n clearly, the line m parallel to an imaginary connecting line along the base A of the membrane 8, 14 through the two oscillating rods 10a and 10b, and line n runs perpendicular to line m. The two oscillating rods 10a, 10b are indicated here by the two crosses. The bending lines along the lines n and m are, in contrast to the embodiment according to FIG Fig. 2 no longer symmetrical. This comes about in particular because the stiffnesses of the membrane 8, 14 differ from one another along the two lines m and n. It is consequently the case, inter alia, that the locations along the base area A of the membrane 8, 14 at which the second derivative of the deflection of the membrane 8, 14 from its rest position is essentially zero, are no longer through a circular line 22, but through an ellipse 23 are given.

For the most efficient energy transfer possible from the rods 15a-15d to the membrane 8, 14, it is correspondingly advantageous to arrange the rods 15a-15d along the ellipse 23 running around the center point M of the base area A of the membrane 8, 14. Then all rods 15a-15d would be arranged in the area of a location at which the second derivative of the deflection of the membrane 8, 14 from a rest position as a function of the location along the base area is essentially zero, i.e. where the rods 15a- 15d each experience a maximum deflection in the area attached to the membrane 8, 14. Furthermore, it is advantageous if the distance between each of the rods 15a-15d and a coil with a core, which is preferably arranged above the center point M of the base area A of the membrane 8, 14, is essentially the same, so that the rods 15a-15d can be made to vibrate evenly. If you want to find the best compromise between these two requirements, those in the figures are an example Figs. 4b-Fig. 4c shown, preferred arrangements for the case of an electromechanical transducer unit 12 with three and four rods 15a-15d.

In the case of three rods 15a-15c, these are arranged in the corner points of an equiangular triangle running around the center point M of the membrane 8, 14, as in FIG Figure 4b shown. The connecting line between two 15a, 15b of the three rods 15a-15c runs parallel to a connecting line between the two oscillating rods 10a, 10b. For the execution according to Figure 4b the two rods 15a, 15b are also arranged at a location along the base A of the membrane 8, 14 at which the second derivative of the deflection of the membrane 8, 14 from the rest position is essentially zero. Alternatively, it is also conceivable that only one of the three rods 15a-15c is arranged at a location along the base area A of the membrane 8, 14 at which the second Derivation of the deflection of the membrane 8.14 is essentially zero. The connecting line between the other two of the three rods 15a-15c then preferably runs parallel to a connecting line between the two oscillating rods 10a, 10b.

How out Figure 4c As can be seen, in the case of four rods 15a-15, the rods 15a-15d are, on the other hand, similar to the case of a membrane 8, 14 as in FIG Fig. 2 , preferably arranged along a circular line around the center point M of the base area A of the membrane 8,14, so that two of the four rods 15a-15d are opposite each other in relation to the center point M of the base area A of the membrane 8,14. As a result, however, in contrast to the configuration according to Fig. 2 For this example, only two of the four rods 15a-15d are arranged at a location along the base surface of the membrane 8, 14 at which the second derivative of the deflection is essentially zero, since these locations describe the ellipse 23.

A device 1 with an oscillatable unit 3 and an electromechanical transducer unit 12 according to the invention is therefore a coupled resonator system with several resonance frequencies, similar to that in the as yet unpublished German patent application with the file number 102015104533.8 described system. For the sake of simplicity, such a coupled resonator system is illustrated below with reference to Fig. 5 in the case of an electromechanical transducer unit 12 with four rods 15a-15d, as in FIG Figure 4c explained. In the case of an odd number of rods, in particular in the case of three rods 15a-15c, similar considerations apply. It should be pointed out, however, that in comparison to an arrangement with an even number of rods 15a-15d, in particular due to the respective symmetries of the arrangements, relatively more complex oscillation modes can occur.

In a coupled resonator system with four rods 15a-15d and a vibratable unit 3 in the form of a tuning fork, three resonance frequencies occur which each belong to an antisymmetrical and two symmetrical vibration modes, as shown in the frequency spectrum recorded in air Fig. 5 evident. The antisymmetrical oscillation mode f1 for this exemplary frequency spectrum is approx. 864 Hz, while the two symmetrical oscillation modes f2 and f3 are at 1050 Hz and at 1135 Hz. In the antisymmetrical oscillation mode with the frequency f1, the rods 15a-15d move towards each other in the area facing away from the membrane 8, 14 when the two oscillating rods 10a, 10b move away from each other in the area of the paddles 11a, 11b. This oscillation mode corresponds to the natural oscillation movement of the tuning fork 3, for example a tuning fork 3, which is used in a LIQUIPHANTEN. In the symmetrical oscillation modes, on the other hand, the two oscillating rods 10a, 10b also move towards each other in the area of the paddles 11a, 11b, when the rods 15a-15d move towards one another in the area facing away from the membrane 8, 14. For the symmetrical oscillation modes with the resonance frequencies f2 and f3, the oscillation amplitude of one of the two pairs of rods 15a and 15b or 15c and 15d is slightly stronger than that of the other pair of rods. If the resonance frequencies of the individual oscillation modes f1-f3 are close enough to one another, however, this is irrelevant and the rods 15a-15d and the oscillating rods 10a, 10b oscillate essentially with the same amplitude.

List of reference symbols

1

Vibronic sensor

2

Sensor unit

3

Vibratory unit

4th

medium

5

container

6th

Drive / receiver unit

7th

Electronics unit

8th

Membrane of the vibratable unit

9

Vibrating element

10a, 10b

Vibrating rods

11a, 11b

paddle

12th

electromechanical converter unit

13th

Housing of the electromechanical converter unit

14th

Diaphragm of the electromechanical converter unit

15a-15d

Poles

16a-16d

Magnets

17th

Kitchen sink

18th

Core of the coil

19th

cup-shaped anchor unit

20th

floor

21st

Circumferential walling

22nd

Circle line

23

ellipse

A.

Base area of the membrane

M.

Center of the membrane

L.

Length of rods

λ

Wavelength of the waves propagating along the rods

Claims (13)

1. Electromechanical transducer unit (12) for a field device (1) used in automation engineering, wherein said unit comprises at least

   - a membrane (14) which can be caused to vibrate mechanically,

   - at least three rods (15a, 15b, 15c) which are fixed by a non-positive connection to the membrane (14) in a manner that is perpendicular to a basic surface (A) of the membrane (14),

   - a housing (13), wherein the membrane (14) forms at least a subsection of a wall of the housing (13), and wherein the rods (15a, 15b, 15c) extend into the housing interior,

   - at least three magnets (16a, 16b, 16c), wherein, in each case, one magnet (16a, 16b, 16c) is fixed to each of the at least three rods (15a, 15b, 15c) in the end area facing away from the membrane (14), and

   - a coil (17) with a core (18), wherein said coil is fixed inside the housing (13) above the magnets (16a, 16b, 16c), and wherein an electrical alternative current signal can be applied to said coil (17),

   wherein the coil (17) is designed to generate a magnetic field, wherein said magnetic field causes the rods (15a, 15b, 15c) to produce mechanical vibrations perpendicular to the longitudinal axis of the two rods (10a, 10b) using the magnets (16a, 16b, 16c),
   wherein the rods (15a, 15b, 15c) are fixed to the membrane (14) in such a way that the vibrations of the membrane (14) result from the vibrations of the rods (15a, 15b, 15c), wherein at least one of the rods (15a, 15b, 15c) is fixed to the membrane (14) at a position (22, 23) along the basic surface (A) of the membrane (14),
   wherein at said position (22, 23) the second derivative of the deflection of the membrane (14) from a rest position, as a function of the position on the basic surface (A), is essentially zero.
2. Electromechanical transducer unit (12) as claimed in Claim 1,
   characterized in that
   at least one of the rods (15a, 15b, 15c) is fixed to the membrane (14) essentially along a circular line (22) extending around the center (M) of the basic surface (A) of said membrane (14).
3. Electromechanical transducer unit (12) as claimed in Claim 2,
   characterized in that
   the number of rods (15a, 15b, 15c) is an even number, wherein said rods (15a, 15b, 15c) are arranged symmetrically along the circular line (22) around the center (M) of the membrane (14).
4. Electromechanical transducer unit (12) as claimed in Claim 2,
   characterized in that
   the number of rods (15a, 15b, 15c) is an uneven number, wherein said rods (15a, 15b, 15c) are arranged in an equiangular manner along the circular line (22) around the center (M) of the membrane (14).
5. Electromechanical transducer unit (12) as claimed in at least one of the previous claims, characterized in that
   the coil (17) with the core (18) is arranged essentially above the center (M) of the basic surface (A) of the membrane (14).
6. Electromechanical transducer unit (12) as claimed in Claim 5,
   characterized in that
   each of the magnets (16a, 16b, 16c) is essentially at the same distance from the coil (17) with the core (18).
7. Electromechanical transducer unit (12) as claimed in Claim 6,
   characterized in that
   the distance between each of the magnets (16a, 16b, 16c) and the coil (17) with the core (18) is less than 2 mm.
8. Apparatus (1) designed to determine and/or monitor at least one process variable of a medium (4) in a vessel (5), wherein said apparatus comprises at least

   - a sensor unit (2) with at least an electromechanical transducer unit (12) as claimed in at least one of the previous claims, and

   - an electronic unit (7),

   wherein the electromechanical transducer unit (12) is designed to excite the sensor unit (2) to produce mechanical vibrations by means of an electrical excitation signal in the form of an electrical alternating current signal which is applied to the coil (17), and to receive mechanical vibrations of the sensor unit (2) and to convert them to an electrical reception signal in the form of an electrical alternating current signal, and
   wherein the electronic unit (7) is designed to generate the excitation signal on the basis of the reception signal and to determine the at least one process variable at least using the reception signal.
9. Apparatus as claimed in Claim 8,
   wherein the sensor unit (2) is a unit capable of vibrating (3).
10. Apparatus as claimed in at least one of the Claims 8 or 9,
    wherein the unit capable of vibrating (3) comprises at least a subsection of the membrane (8, 14), or at least a subsection of the membrane (8, 14) and at least a vibrating rod (10a, 10b) fixed on the membrane.
11. Apparatus as claimed in at least one of the Claims 8 to 10,
    wherein the process variable is defined by a level or the flow of the medium (4) in the vessel (5), or by the density or the viscosity of the medium (4).
12. Apparatus as claimed in at least one of the Claims 8 to 11,
    wherein the unit capable of vibrating (3) is a tuning fork with two vibrating rods (10a, 10b), wherein the electromechanical transducer unit (12) comprises four rods (15a-15d), and wherein two (15a, 15b) of the four rods (15a-15d) of the electromechanical transducer unit (12) which are fixed to the membrane (8, 14) and the two vibrating rods (10a, 10b) fixed to the membrane (8, 14) are in mirror symmetry to one another in relation to the plane perpendicular to the longitudinal axis through the rods (15a, 15b) and/or are arranged opposite the vibrating rods (10a, 10b).
13. Apparatus as claimed in at least one of the Claims 8 to 11,
    wherein the unit capable of vibrating (3) is a tuning fork with two vibrating rods (10a, 10b), wherein the electromechanical transducer unit (12) comprises three rods (15a-15c), and wherein the three rods (15a-15c) are arranged in the corners of an equilateral triangle extending around the center (M) of the membrane (8, 14) in such a way that the connecting line between two (15a, 15b) of the three rods (15a-15c) is parallel to a connecting line between the two vibrating rods (10a, 10b).

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